1. Field of the Invention
This invention relates to technology for applying a coating to at least upper surfaces or the lower surfaces of rings having upper and lower surfaces, an inner circumferential surface and an outer circumferential surface and having a gap forming a pair of end surfaces, and is preferably utilized for instance, to coat piston rings, snap rings, and the like.
2. Description of the Related Art
The application of a coating of synthetic resin containing solid lubricant to the lower surface, or the upper and lower surfaces, of a piston ring is performed on certain piston rings to attain the objectives listed below.
1) To prevent the piston ring material, aluminum, from adhering to the piston ring surface. PA1 2) To prevent roughing of the piston ring groove. PA1 3) To alleviate wear on the piston ring groove. PA1 4) And to improve break-in conformity between the piston ring groove and the upper and lower surfaces of the piston ring. PA1 1) Since the piston rings 70 cannot be placed on the metal net 60 without spaces, much coating material misses the piston ring 70 and wastefully adheres to the metal net 60. PA1 2) Much labor time is required to arrange the rows of the piston rings 70. Also, much time is required to reverse the piston rings 70, when coating the upper and lower surfaces of the piston rings 70. PA1 3) Since the spaces between the piston rings 70 and the adjacent piston rings 70 are not uniform, the adherence of coating material in the circumferential direction of the piston ring 70 is irregular, and in particular the thickness of the coating film on the outer circumferential surface is irregular. PA1 4) Non-uniform temperatures occur between portions being in contact with the metal net 60 and other portions not being in contact with the metal net 60 in the piston ring 70, since the piston ring 70 is placed on the metal net 60. These non-uniform temperatures cause variations in the adhesion of the coating film. PA1 5) Foreign matter, dust or scratches tend to be present on the coating film formed on the piston ring 70 when the piston ring 70 on the metal net 60 is reversed. PA1 6) During arrangement, the reversal process or conveyor transport of the piston rings 70 on the metal net 60, the piston ring 70 tends to move around on the metal net 60 and the following problem occurs. Namely, the piston rings 70 pile atop one another, causing uncoated portions to occur in the surface of the piston ring 70. Also when undried rings make contact with undried rings or the net holes, metal-to-metal contact occurs locally, causing burrs to occur in the coating film and causing variations in the thickness of the coating film. PA1 7) The openings in the metal net 60 gradually become smaller as the coating process is repeated, making frequent replacement of the metal net 60 necessary.
A coating film formed on the lower surface, or the upper and lower surfaces of the piston ring generally, has a thickness of 5 to 10 .mu.m and this film is required to have good adhesion, no damage, no intrusion of foreign matter, and a uniform thickness. In many cases, adherence of coating material to the outer circumferential surface of the piston ring is overlooked.
The conventional method for covering the upper and lower surfaces of a piston ring with a synthetic resin containing solid lubricant will be hereinafter described.
As shown in FIG. 10, a plurality of piston rings 70 are placed onto the surface of a metal net 60. The size of the metal net 60 is approximately 500 mm.times.400 mm and the size of the holes in the net is approximately 5 mm.times.5 mm. Approximately fifty piston rings 70 can be placed onto the metal net 60. Many piston rings 70 are placed on the metal net 60 with as little space between them as possible, in order to reduce wasteful coating material. The piston rings 70 are arranged by hand onto the metal net 60 in rows, requiring a large amount of time.
The piston rings 70 loaded on the metal net 60 are transported to the preheating oven by conveyor and preheated. Next, the piston rings 70 are sent to the coating booth provided with a coating robot. The coating robot coats the piston rings 70 by moving a coating gun above the metal net 60 in a lattice pattern. When coating of one side of the piston ring 70 is complete, the piston rings 70 are sent to a pre-baking furnace and pre-baking is performed. When coating the upper and lower surfaces of the piston ring 70, the piston rings 70 on the metal net 60 are reversed and the above process is then repeated once again. When pre-baking is complete, the piston rings 70 are sent to a baking furnace and baked completely.
However the above method has the following disadvantages.